The subject invention generally pertains to fasteners, and more specifically to screws and the threading thereof.
U.S. Pat. No. 6,666,638 issued to Craven teaches a fastener for securing remnant-producing materials, a method of manufacturing the fastener, and a method of using the fastener so that remnants or slivers produced by rotation of the fastener are substantially embedded within the remnant-producing material. The fastener has two separate threaded portions, a first or lower threaded portion having a first thread pitch and a second or upper threaded portion having a second thread pitch. The leading lower thread portion is designed to engage the work piece more firmly than the following upper thread portion. Thus, upon insertion of the screw, as both threaded portions engage the work piece, the first section substantially maintains its position or insertion rate with respect to the work piece defined by the pitch of the lower threads. The second threaded portion having a different thread pitch engages the work piece less firmly and thus is pulled through the work piece by the first set of threads substantially at the rate defined by the angle of the first set of threads. If the respective upper and lower thread pitches were equal, the upper threads would merely follow in the tracks or parallel to the tracks of the first leading threads. By making the thread pitches different, the second threaded portion engages and pulls with it remnants and/or wall material, retaining or pulling it into the bore. The pitch of the second portion threads is less than the pitch of the threads of the first portion. The second threaded portion pitch may spiral in the same direction as the first thread portion, may be of zero pitch, i.e., one or more rings around the screw shank, or may be of negative pitch, i.e., spiral in a direction that is opposite of the threads on the first or lower threaded portion. The threads on the second or upper threaded portion have a larger diameter than the threads on the first or lower threaded portion. In use, the first or lower threaded portion of the fastener is engaged with the remnant-producing material and rotated in the direction of the threads to insert the fastener into the material. This rotation may produce remnants or slivers by extruding, cutting, or some other mechanism. These remnants may extend from the hole in the material or workpiece surface made by the screw. After the lower threaded portion is completely inserted into the material, the upper threaded portion of the fastener enters the material. Because the upper threaded portion has threads that have a different thread pitch relative to the threads on the lower threaded portion, the threads on the upper threaded portion capture the remnants that have been extruded within and/or onto the surface of the workpiece. As the fastener is completely inserted into the material, the remnants that have been extruded by the lower threaded portion of the fastener are substantially retained in the bore by the upper threaded portion of the fastener.
U.S. Pat. No. 6,616,391 issued to Druschel discloses a screw incorporating a sharp point angle, undercut head and a boring portion to contain displaced decking material below the screw head. As a result, a single-step installation produces a neatly countersunk appearance. The screw head includes a circumferential undercut connected to the top surface of the screw head by a periphery oriented parallel to the screw axis. Converging conical surfaces define the v-shaped undercut that connects the periphery of the screw head to the shank of the screw. The junction of the periphery with the radially outer conical surface of the undercut forms a peripheral lip. In an alternative embodiment, this lip may be provided with unidirectional cutting teeth that initially improve countersinking and later help prevent the screw from backing out. What would typically constitute an unthreaded portion of a deck screw (extending from the head to the threaded portion of the shank) is replaced with at least one radially projecting helical flute. The helical flute has a helical orientation opposed to that of the screw thread. The helical flute forms a boring portion that, during screw installation, pushes displaced deck material away from the surface of the decking while opening a larger diameter hole beneath the screw head. Decking material displaced by countersinking the screw head is contained beneath the head by the undercut head configuration. The material contained beneath the screw head during installation at least partially fills the hole reamed by the boring portion. A precisely defined sharp point angle and a sharp thread combine to help prevent material creep up the screw shank.
U.S. Pat. No. 6,644,904 issued to Chen et al. discloses a screw structure to be used for fastening wooden plates where the bottom end thereof can make a pointed or planar cut at a certain degree so as to reduce the friction and to enable a more efficient and smoother fastening. The aforesaid screw structure comprises a shank with a screw head at the top end and square edged threads arranged helically along the shank; wherein, between the third and the fourth threads from the bottom end of the screw, a shaft flange is positioned parallel or slanted to the longitudinal axis of the shank. When the screw is driven, the shaft flange makes a pointed or planar cut the object to be secured and thereby reduces friction. The abovementioned screw structure positions the shaft flange between the upper and the lower rims of the third and the fourth threads. The screw structure has a shaft flange disposed at the bottom end of the shank in a convex trapezoid shape to facilitate a pointed or planar cut.
None of the above prior art teaches or suggests a screw having a thread diameter consistent along the entire shaft while including four different thread portions: first major threads (preferably eleven) at the tip end of the screw shaft, second major threads (preferably three) at the head end of the screw shaft with substantially the same thread pitch as that of the first major threads, reverse threads (preferably five) separating the first major threads and the second major threads and angled substantially opposite therefrom, and a helical ridge within the screw shaft portion that has the reverse threads (and preferably spanning three of the five reverse threads).
The above thread configuration of the screw of the subject invention functions to sever the plastic, composite or other remnant producing material with the helical ridge portion, which also serves a torque reduction function. The alternating sections consisting of first and second major threads and reverse threads contain the remnants and debris within the screw hole, while the consistent thread pitch of the first and second major thread portions and the consistent thread diameter along the entire shaft provide uniform screw fastening properties.